The optimal diet to prevent cardiovascular disease (CVD) and diabetes is uncertain. Reducing saturated fat and transunsaturated fat lowers cardiovascular risk. However, within the context of low saturated and transunsaturated fat, diets can vary widely in other energy-providing nutrients, particularly carbohydrate. Some authorities advocate reducing carbohydrate and replacing it with fat and protein. It is hypothesized that such dietary changes will improve blood lipids, insulin resistance, and blood pressure. It has also been argued that slowly absorbed carbohydrates, i.e. those with a low glycemic index, have beneficial effects on these risk factors for CVD and diabetes. Supportive evidence comes from small scale studies, which in aggregate have been insufficient to guide nutrition policy. We propose to conduct a large-scale, controlled dietary trial that examines the effects of reducing carbohydrate from a high level (58% kcal) to a low level (43% kcal) and of lowering glycemic index from a high level (>85) to a low level (<55) on the major established diet-related CVD risk factors as well as insulin sensitivity, strongly linked to type 2 diabetes. Trial participants (n=160, approximately 50% women, 50% African-American) will have elevated blood pressure and a high prevalence of other CVD risk factors such as dyslipidemia and overweight or obesity. The core design is a randomized controlled feeding study that uses the same methodology as the DASH and OmniHeart trials. Four diets will be tested for 5 weeks each: higher or lower carbohydrate, each with higher or lower glycemic index. The participants will be maintained in the weight-stable state. The primary endpoints will be systolic blood pressure;plasma LDL cholesterol, HDL cholesterol and triglycerides;and insulin sensitivity determined directly by intravenous glucose tolerance test. Secondary endpoints will include diastolic blood pressure;atherogenic lipoprotein particle types related to dyslipidemia of the metabolic syndrome;fructosamine;and overall CVD risk estimated by standard risk equations. The primary diet contrast will be high carbohydrate, high glycemic index vs low carbohydrate, low glycemic index. Other contrasts will aim to distentagle level of carbohydrate and glycemic index;i.e. the effects of higher vs lower glycemic index at each carbohydrate level, and the effects of high vs low carbohydrate level at each glycemic index. The objective of this type of "feeding study" is to establish definitively the biological effects of dietary carbohydrate level and type on risk of CVD as well as insulin resistance. We expect that the results will be immediately applicable to public health and clinical guidelines, and will influence nutrition policy.